(1) Field of the Invention
This invention relates to a substrate heat treatment apparatus for heat-treating substrates such as semiconductor wafers, glass substrates for liquid crystal displays, glass substrates for photomasks and substrates for optical disks (hereinafter simply called “substrates”). More particularly, the invention relates to a technique for heat-treating a substrate as sucked in a position slightly spaced from a heat-treating plate.
(2) Description of the Related Art
With an increasingly fine line width of patterns formed on substrates today, the requirements for line width uniformity have become stringent, which has led to a strong demand for temperature uniformity in baking treatment in photolithography, especially in baking treatment after exposure (PEB: Post Exposure Bake). However, with enlarged substrate sizes, increased curvatures of substrates take place in the semiconductor manufacturing process. It is difficult to satisfy the requirements for temperature uniformity in a proximity heating mode that heats each substrate only by placing the substrate as separated by a minute space from a heat-treating plate.
Thus, a suction bake mode has been proposed in order to perform uniform heat treatment even for curved substrates. This type of apparatus includes a heat-treating plate with a heater, support elements and a sealer arranged on the upper surface of the heat-treating plate, and exhaust bores formed in the upper surface of the heat-treating plate.
The support elements are in the form of bulges and dimples formed by machining the upper surface of the heat-treating plate (as disclosed in Japanese Unexamined Patent Publication H2-290013 (1990), for example), or metallic projections formed on the upper surface of the heat-treating plate and coated with resin (as disclosed in Japanese Unexamined Patent Publication H10-284360 (1998), for example). The sealer is in the form of a ring disposed in a position for contacting edges of a substrate. With these apparatus, the substrate is sucked as the sealer closes lateral areas of a space formed between the heat-treating plate and the substrate supported by the support elements, and gas is exhausted from this space through the exhaust bores. By sucking the substrate in this way, any curvature of the substrate is corrected whereby the substrate is heated uniformly.
The conventional apparatus noted above have the following drawbacks.
In the former construction, the bulges and dimples are formed of the same material as the heat-treating plate, and have a much higher thermal conductivity than air. Therefore, the thermal resistance between the bulges and the substrate in direct contact is much smaller than the thermal resistance between the dimples and the substrate not in direct contact but interposed by space. In time of heat treatment, therefore, heat is transferred faster to the parts (contact parts) of the substrate in contact with the bulges than to the parts (non-contact parts) of the substrate out of contact with the bulges. The rate of temperature increase differs greatly between the contact parts and non-contact parts, causing serious variations in heat history over the substrate surface. As a result, a pattern of uniform line width cannot be formed on the substrate.
In the latter construction, surfaces of the projections are coated with resin in order to prevent metallic contamination. Since the interiors of the projections are metal with high thermal conductivity, heat is positively transferred to the parts (contact parts) of the substrate in contact with the projections. On the other hand, heat transfer to the parts (non-contact parts) of the substrate out of contact with the projections takes place through a gas layer, and is relatively small in amount. This results in the inconvenience of the rate of temperature increase differing greatly between the contact parts and non-contact parts, causing serious variations in heat history over the substrate surface.